Asymmetric hip stem

ABSTRACT

An improved asymmetric femoral hip stem component for use in cementless hip replacement procedures is described. The stem comprises a proximal region having a novel three-dimensional configuration to allow for better fit and stability of the stem within the femoral intramedullary canal with minimal removal of strong bone therefrom. The stem further includes a twist isolated in the mid-stem region to improve fit while minimizing enlargement of the femoral canal. A distal end comprising a rotated internal slot is also described, wherein the slot reduces bending stiffness of the stem in both the coronal and sagittal planes.

I. BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to femoral hip prostheses, and moreparticularly to a femoral hip stem component having a shape whichprovides a better fit within the femoral medullary canal.

2. Background of Invention

Hip arthroplasty procedures involve the implantation of a prostheticstem component within a femoral intramedullary canal. A ball on theproximal end of the stem cooperates with a socket of an acetabulum toprovide for articulation between the femur and the acetabulum. In orderto maintain pain-free articulation of the hip joint followingimplantation of the stem, it is important that the stem be securelyfastened to the intramedullary canal. Such fastening can be accomplishedwith a bone cement which adheres to the stem and the wall of theintramedullary canal. In addition numerous stems have been provided witha porous surface as taught in U.S. Pat. Nos. 4,550,448 (Kenna) and3,605,123 (Hahn) to either accommodate adherence with the bone cement orenhance a press fit between the porous surface and the wall of theintramedullary canal. If a press fit is desired with the intramedullarycanal, the stem contour should closely match the contour of theintramedullary canal so that the porous surface is in intimate contactwith bone, thereby enabling bone to grow into the porous surface.

Various patents relate to a femoral component for press fit with, andbiological fixation to, the wall of the proximal metaphysis andintramedullary canal. U.S. Pat. Nos. 4,589,883 (Kenna) and 4,738,681(Koeneman et al.) teach a femoral stem having a twist in the proximalregion for improved fit and stability within the femoral canal. Whilethis appears to be close to the geometry of the natural femur, inpractice the rotational motion of the stem induced by the twist can leadto enlargement of the implantation site and the formation of gaps at theimplant/bone interface. The twist in this region also prevents the stemfrom sitting within the neck of the femur. Instead, the stem sits in thebone in a rotated position, thus making preparation of the implantationsite more difficult since surgeons often attempt to change therotational position of the implant to restore the normal position of thefemoral head.

A second limitation existing in the art is the cross-sectional shape ofthe stem, in particular in the proximal regions of the stem, wherein thegeometry often necessitates the removal of strong bone within the femur(e.g. the calcar femorale and the medial border of the greatertrochanter) before the stem can be correctly implanted, achieving aclose fit to the canal. In practice, this is difficult to achieve withexisting surgical instruments, so many such prostheses are difficult toimplant without undersizing or misalignment. U.S. Pat. No. 5,358,534(Dudasek, et al.), as well as Kenna described above, teach a stemwherein a transverse cross-section taken in the proximal region of thestem is substantially rectangular in shape (i.e. it has parallelanterior and posterior edges). Such a shape does not conform well to theinternal anatomy of the femoral intramedullary canal and necessitatesthe removal of the bone from the greater trochanter during implantation.In addition, while the Dudasek stem does disclose the presence of aposterior concavity to clear the posterior cortex of the intramedullarycanal, it does not teach or suggest the geometry or dimensions of theconcavity for allowing the medullary cavity to be maximally filled withthe stem without the need to remove the calcar femorale.

U.S. Pat. No. 4,813,963 (Hori, et al.) is directed to a stem having aconfiguration that, according to the specification, more accuratelyreflects the anatomic contour of the intramedullary canal. Inparticular, the patent teaches a stem wherein the proximal portion, intransverse cross section, has an asymmetric contour to define ananterior side which forms an acute angle with a lateral side, and aposterior side which approaches the anterior side in the medialdirection. In addition, the medial side is arcuate in shape while theother sides have linear edges. However, this stem still requires removalof bone from the greater trochanter due to the wide angle of theanterior/lateral edge and the bulk in the posterior/lateral corner.Moreover, although designers of previous prosthetic devices haveutilized simple cross-sectional shapes with flat sides to facilitatemanufacture of these implants, the inner contours of the femur arearcuate and rarely linear. Consequently, stems such as that taught byHori, et al. only achieve contact with the femoral cavity at discretepoints, typically along relatively sharp edges. This may result inlocalized stem concentration and could lead to an increased incidence ofbone fracture during implantation. In addition, areas of relatively softbone will be left between the localized points of contact. These areastend to become osteoporotic with time, leading to less biologicalattachment of the prosthesis through bony ingrowth. These areas may alsoact as open channels within the bone structure, making the femursusceptible to infiltrating particles generated by wearing of theartificial joint. The biological reaction to these particles quitefrequently leads to erosion of bone around the prosthesis and may causeloosening and failure of the implant.

A third problem with the prior art is thigh pain that is oftenexperienced after cementless total hip arthroplasty. This pain iscommonly linked to the stiffness mismatch between the bone and theprosthesis. Provision of a distal slot reduces bending stiffness,thereby reducing subsequent thigh pain. Other beneficial effects ofreducing distal bending stiffness include easier stem insertion, lowerincidence of distal fractures during stem insertion, and greater ease ofimplanting the correct size implant. Conventional slotted prostheses,such as those of Thongpreda et al. (EPO 0 543 099A2) and Noiles (U.S.Pat. No. 3,996,625) have slots oriented in the coronal plane, andtherefore only provide the foregoing benefits in one plane of bending.In most activities, the force acting on the distal stem is not directedanteriorly or posteriorly, but has a significant medial-lateralcomponent.

II. SUMMARY OF THE INVENTION

In light of the foregoing problems with prior art femoral hip stemsdesigned for cementless hip arthroplasty procedures, it is an object ofthe present invention to provide stability of the implant/bone interfacein resisting forces applied to the femoral head by optimizing the fitbetween the femoral cortex and the surface of the implant. Specifically,it is an object of the present invention to provide an implant having asurface contoured to match the cortical walls of the femur (a)anteriorly and posteriorly at the level of the femoral neck osteotomy;(b) anteriorly and medially at the level of the lesser trochanter; and(c) anteriorly, posteriorly, and laterally within the femoral mid-stem.

Another object of the present invention is to provide a femoral implantthat is contoured to allow for minimal removal of bone duringimplantation, in particular the bone from the greater trochanter and thecalcar femorale.

Another object of the present invention is to provide for maximum easeof insertion of the implant into the femur without undersizing ormisalignment.

Still another object of the present invention is to minimize theenlargement of the implantation envelope during insertion of the stem.

Finally, another object of the invention is to minimize bendingstiffness of the stem in both the sagittal and coronal planes, therebyreducing the incidence of thigh pain, allowing for easier steminsertion, and reducing the incidence of distal fractures during steminsertion, for example.

These and other objects of the present invention are achieved byproviding a femoral component for use in hip arthroplasty proceduresthat is designed to fit closely the anatomical contours of the femoralintramedullary canal with minimal bone removal. In particular, certainaspects of the present invention are directed to a femoral stemcomprising:

(a) a distal region having a distal portion, a center, and alongitudinal axis intersecting the center, the axis further defining theanterior, posterior, medial and lateral faces;

(b) a proximal region including a top end portion; and

(c) a mid-stem region positioned between the distal and proximalregions;

wherein the proximal region is configured such that a cross sectiontaken perpendicular to the longitudinal axis includes:

(d) a substantially arcuate anterior side having a varying radius ofcurvature;

(e) a posterior side having a concavity;

(f) a substantially arcuate medial side; and

(g) a substantially arcuate lateral side that slopes anteriorly todefine an angle of declination.

The proximal cross section preferably includes a substantially arcuateposterior/lateral corner located such that the distance from the surfaceof the femoral component to the distal longitudinal axis always exceedsa minimum distance where said distance is larger than or equal to thediameter of the drive shaft of a rigid drill or reaming instrument whichmay be used to machine the medullary canal.

Certain aspects of the present invention further include two adjacentgeometric bodies, a femoral neck segment and a medullary segment, whichas best illustrated in transverse cross sections of the proximal region,are oriented at an acute angle with respect to one another.

The present invention is also directed to a femoral stem having aposterior concavity positioned in the proximal region of the stem,wherein the concavity is configured to preserve the calcar femoralepresent in the region of the lesser trochanter. Other features of theinventive stem include a cant or taper on the lateral face of theproximal region of the stem in the medial direction which, incombination with the transverse cross section geometry of the proximalregion, allows for minimal bone removal from the greater trochanter.

Other aspects of the present invention include the presence of a stemtwist about the longitudinal axis, most preferably restricted to themid-stem region, such that a constant twist angle exists in the proximalregion of the stem. Isolation of the stem twist in this region asopposed to the proximal region minimizes enlargement of the implantationenvelope caused by such twisting while at the same time providesimproved implant fit and stability upon implantation.

The present invention is also directed to a femoral stem having aninternal distal slot that is rotated to the coronal plane, preferablyabout 30 degrees, to reduce the bending stiffness of the stem in boththe sagittal and coronal planes.

The foregoing objects and features of the invention, as well as otherobjects and features, will become more fully apparent when the followingdetailed description of the invention is read in conjunction with theaccompanying drawings.

III. BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1A are an anterior and medial views, respectively, of a leftfemoral stem.

FIG. 1B is a transverse cross-section of the stem taken along lines1B--1B in FIG. 1.

FIGS. 2 and 2A are anterior and medial views, respectively, of a solidmodel for which the three-dimensional shape of the femoral stem isderived.

FIG. 2B is a transverse cross-section of the solid model taken alonglines 2B--2B in FIG. 2.

FIG. 3 is an anterior view of a left femoral stem illustrating severaltransverse cross-sections A--A through J--J.

FIGS. A--A through FIGS. J--J are enlarged views of the respectivecross-sections taken in FIG. 3.

FIGS. 4 and 4A are anterior and medial views, respectively, of a leftfemoral stem illustrating the medullary and femoral neck segments of thestem.

FIG. 4B is a transverse cross-section taken along line 4B--4B in FIG. 4.

FIG. 5 is a posterior view of a left femoral stem seated within thefemur.

FIG. 5A is a transverse cross-section view of the stem/femur taken alonglines 5A--5A in FIG. 5.

FIG. 6 is a medial view of a left femoral stem seated within a femur.

FIG. 7 is a top plan view of the femoral stem.

FIG. 8 is another anterior view of the left femoral stem.

FIG. 9 is a bottom plan view of the femoral stem.

FIG. 10 is a transverse cross section view of the distal region of thestem taken along lines 10--10 of FIG. 9.

FIG. 11 is a schematic side view of the stem illustrating the ellipticalconfiguration of the distal end.

IV. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to am improved asymmetric femoral stemfor implantation into a proximal femur. More particularly, certainaspects of the invention are directed to a femoral stem that comprisesseveral key features that provide an optimum balance of the followingfactors:

(a) stability of the implant/bone interface in resisting forces appliedto the femoral head;

(b) minimum removal of bone during implantation;

(c) maximum ease of insertion of the prosthesis into the femur withoutundersizing or misalignment; and

(d) minimum enlargement of the "implantation envelope" during insertionof the stem.

Referring now to the figures, a left femoral stem component (10) isillustrated and comprises a proximal region (20), a distal region (40),and a mid-stem region (30) positioned therebetween, with each regionmost preferably comprising about one-third of the total length of thestem (FIG. 1). The total length (H1) of the stem (10) preferably rangesfrom about 4.5 inches to about 6.5 inches (FIG. 1A). The stem furtherincludes a longitudinal or distal axis (Z) which corresponds with thedistal center (5) in the distal region (40) (See FIGS. 3 and FIGS. A--Athrough J--J). Unlike some conventional stems, the posterior face (P) ofthe proximal region, as shown in FIG. 1A, has only a slight bowgenerally extending in the proximal-distal direction, with a relativelylarge radius (R) measured from the anterior face (A), as shown in FIG.1A, thereby minimizing the potential for enlargement of the femoralcavity upon implantation. Moreover, the overall design of the implant,which is discussed in greater detail below, allows for a higher femoralneck osteotomy, most preferably about 5 mm higher than for conventionalhip replacement procedures (i.e. about 18 mm above the lessertrochanter).

The inventive stem may be fabricated from conventional materials, mostpreferably cobalt-chromium-molybdenum alloy (VITALLIUM by Howmedica,Inc.). In a preferred embodiment, the final finish of the implant issatin except in the tapered trunion (22) and between points 3 and 4, asshown in FIG. 1 (i.e. comprising the entire distal region and portion ofthe mid-stem region), both areas of which have a bright polish finish(FIG. 1).

For ease of explanation, the remaining disclosure is divided into threesections. Section I is directed to the proximal region of the inventivestem, Section II is directed to the mid-stem region, and Section III isdirected to the distal region of the stem. It should further be notedthat while the description of the invention and the related figures aredirected to the left femoral component, the present invention is alsoapplicable to a right femoral component, which is merely a mirror imageof the left component described and illustrated herein.

I. Proximal Region

Referring now to FIGS. 1 and 1A, the proximal region (20) comprisesapproximately the upper one-third of the stem and is preferably coveredwith a porous coating (discussed in more detail below). The preferredlength of the proximal region ranges from about 1.5 inches (for a stemhaving a total length of about 4.5 inches) to about 2.5 inches (for astem having a total length of about 6.5 inches). The proximal region isfurther divided into a proximal section (23) including a top end portion(21) and neck (22a), a central section (24), and a distal section (25).

The inventive stem was derived from a solid model (10¹), as illustratedin FIGS. 2, 2A, and 2B, which was developed to represent thethree-dimensional shape of the object that would provide optimum fit tothe femoral cavity.

The solid model (10¹) has a flat top surface (21a) that is located atthe same level along its longitudinal axis (Z) as the top surface (21)of the prosthesis. The shape of the inventive stem (10) matches theshape of the solid model (10¹) below the level of the femoral neckosteotomy. The shape of the stem is derived from the solid model byremoving all material above the sloped upper surface (21b) correspondingto the femoral neck osteotomy. The prosthetic neck (22a) (FIG. 1) isthen added to the sloped surface (21b) and its surface contours areblended into those of the sectioned solid model. Additional surfacefeatures may be added to the solid model to modify the bending stiffnessof the distal stem or to accommodate the presence of the porous coating,as discussed further below.

The transverse cross-sectional shape of the solid model (10¹) and stem(10) varies continuously from proximal to distal, as shown in FIGS. A--Athrough J--J, which illustrates typical cross-sections through theinventive implant. This shape was derived to contact several key areasthat are critically positioned in three-dimensional space to maximizecontact between the implant and the femoral cortex, in particular in theregion of the greater trochanter where the removal of bone may beminimized.

Specifically, the shape of the solid model may be defined inthree-dimensional space by a series of profiles defined by anterior,posterior, lateral, and transverse projections of the surface of theimplant. The transverse shape of the solid model, as shown in FIG. 2B,for example, was developed from iterative trials using prototypes thatwere implanted into cadaveric femora and sectioned transversely.Specifically, FIG. 2B illustrates the top-end view section of the solidmodel, wherein dimension (X) represents the lateral/medial width of thesection and dimension (Y) represents the anterior/posterior width of thesection. The proximal cross section comprises a substantially arcuatelateral side (L1) that slopes in the anterior direction to form an angleof declination (α₁). This "sloping" geometry of the lateral side allowsa stem of maximum medial/lateral width (X) to be implanted into thefemur without the necessity of resectioning the medial wall of thegreater trochanter for easier insertion of the stem within the bone foran improved fit therein. Most preferably, the sloping of the lateralside (L1) may be defined by a line (a) having tangential contact withthe lateral side at its lateral-most point (B), wherein the line (a) ispositioned parallel to the sagittal plane. The lateral side (L1) slopesanteriorly to have tangential contact with a second line (b) at a secondpoint (C), such that line (a) intersects line (b) to form an acute angleof declination (α₁) therebetween. Preferably, this angle is from about10 degrees to about 35 degrees in the lateral-medial direction, mostpreferably about 20 degrees.

The cross-section in FIG. 2B further includes a substantially arcuateanterior side (A1) having a varying radius of curvature displacedlaterally. Preferably, the anterior side is configured such that it hastangential contact with a line (c) at point (D) such that line (c) isparallel to the coronal plane and is thus perpendicular to line (a). Theanterior side slopes medially to form tangential contact with a line (d)through point (E) and perpendicular to line (b), such that line (d) isoriented at an acute angle (α₂) with respect to line (c). Preferably,the angle (α₂) is from about 10 degrees to about 30 degrees, mostpreferably about 20 degrees. Typically, point (E) is located a distance(X¹) lateral to the medial-most point of the cross-section (I), whereinX¹ is equal to 0.30X (i.e. 30% of the medial/lateral width).

The cross-section in FIG. 2B also includes a substantially arcuatemedial side (M1), preferably defined between points G and F, and havinga radius (R2) as well as a medial-most point I. Also present is asubstantially non-convex, preferably planar, posterior side (P1), and asubstantially arcuate posterior/lateral corner (27) defined betweenpoints A and B and having radius (R1). Alternatively, the posterior sidemay be concaved. Further, in the preferred embodiment, the center of thecircle defining the posterior/lateral corner (27) is located at thepoint of intersection between the top surface of the solid model (21a)and the distal longitudinal axis. The radius of the corner (R1) is largeenough to allow a rigid reamer to be placed in the intramedullary canalsuch that the longitudinal axis of the reamer and the canal coincidewithout a gap subsequently being formed in the vicinity of theposterior/lateral corner. This necessitates that the radius (R1) belarger than, or equal to, the diameter of the drive shaft of the rigidreamer.

The key level for fit and stabilization of the stem within the femurextends from the level of the femoral neck osteotomy to the base of thelesser trochanter. The transverse cross-sectional shape of the stem inthis region of the bone may be described by a series of lines and arcsas shown in FIG. 1B. Cross-sections taken at this level along thelongitudinal axis are similar in configuration to the more proximalcross-sections of the solid model illustrated in FIG. 2B and include asubstantially arcuate anterior side (A2), substantially arcuate medialside (M1) defined between point G and F, and a lateral side (L1) thatslopes anteriorly to define an angle of declination (α₁) as describedabove in FIG. 2B. The posterior/lateral corner (27) is defined by pointsA and B and has a radius (R1) that is somewhat larger than that presentat the more proximal levels (as illustrated in FIG. 2B), and preferablyis larger than the radius (R2) of the medial side (M2).

Another particularly useful aspect of the stem in this region is theinclusion of a small posterior-medial transition region (28) extendingfrom the posterior side (P2) at point (H) to the medial radius at point(G). As illustrated, this transition region (28) has a radius largerthan that defined by the medial side (M2). The transition region mayfurther be approximated by a line (e) intersecting points H and G andinclined at an angle (α₃) ranging from about 5 degrees to about 15degrees in the medial/lateral direction (i.e. relative to line f), mostpreferably about 8 degrees. This latter feature is particularlypreferred since it allows the stem to fit the profile of the femur bothwithin the base of the femoral neck and within the medial arc.Conversely, use of a single arc to connect points H and F leads to astem profile that contacts the cortical wall of the femur at one pointwithin the posterior-medial corner. This leads, potentially, to an areaof localized stress concentration and increased risk of femoralfracture.

The proximal cross section of the stem further includes an overallconfiguration that can be defined by an axis (X2) bisecting thecross-section through the anterior and posterior sides, wherein the axis(X2) is an equidistance from the lateral-most point (B) and from themedial-most point (I), as illustrated in FIG. 1B. The resulting twosections defined by axis (X2) have different total areas, with thesection including the lateral side (L) of the stem having a larger areathan that of the adjacent section containing the medial side (M).

The foregoing geometry illustrated in both FIGS. 1B and 2B, inparticular the shape of the lateral, medial, and anterior sides, and theposterior/lateral and posterior/medial corners, for example, allows thestem to avoid strong bone in the greater trochanteric region, therebyminimizing bone removal in this area of the intramedullary canal forimproved stability. Moreover, the posterior side (P2) includes aconcavity (29), as illustrated in FIG. 1B, for example. The concavitygenerally begins below sectional line A--A, to and including,cross-sections E--E (as illustrated in the corresponding figures), andis designed to allow retention of the strong bone of the calcar femoraleat the level of the lesser trochanter, thereby increasing the ease ofinsertion of the implant as well as the resistance of the femur torotational deformation. This further facilities implantation of thelargest possible prosthesis that the medullary cavity can accommodate,thus preventing undersizing which is the most common cause of implantfailure. The preferred depth (D1) of the concavity increases from theproximal-most section to the central section of the proximal region, andthen decreases again. This depth (D1) ranges from about 0.1 mm (in theproximal (23) and distal sections (25)) to about 3.0 mm (in the centralsection (24)).

The foregoing transverse configurations also illustrate anotherpreferred aspect of the inventive stem, specifically two intersectingsegments corresponding to two adjacent geometric bodies: a femoral necksegment (S) and a medullary segment (T) (FIGS. 4 and 4A). In thetransverse plane, as shown in FIG. 4B, the medullary segment (T) ispositioned at an angle (θ) relative to the femoral neck segment, asdiscussed in more detail below. Such positioning allows the implant tocontact the following areas simultaneously: anteriorly and posteriorlyat the level of the femoral neck osteotomy; anteriorly and medially atthe level of the lesser trochanter; and anteriorly, posteriorly, andlaterally within the femoral mid-stem.

Specifically, the femoral neck segment (S) comprises a triangularportion (21) of the proximal region and extends from the top end portionto the distal section (25) of the proximal region along, and including,the medial face (M) of the stem. This is best illustrated in FIGS. 4 and4A, wherein the lined region indicates the femoral neck segment (S), andthe remaining portion of the stem comprises the medullary segment (T).The femoral neck segment (S) is further configured such that atransverse cross section taken perpendicular to the distal orlongitudinal axis (Z) comprises a section (lined region) correspondingto the femoral neck segment (S) that is preferably elongated, mostpreferably generally elliptical, and includes a minor axis (S1) passingthrough the section and intersecting the anterior (A) and posterior (P)sides. The section further includes a major axis (X) perpendicular tothe minor axis (S1) that passes through the medial (M) and lateral (L)sides.

The medullary segment (T) is the remaining portion of the proximalregion positioned adjacent to the femoral neck segment and extends from,and includes, a part of the top end portion (21) and lateral face (L),and the distal (40) and mid-stem (30) regions of the stem, asillustrated in FIGS. 4 and 4A (unlined region). The medullary segment(T) is further configured such that a transverse cross section takenperpendicular to the distal or longitudinal axis (Z) also comprises asection similar to that of the femoral neck segment, including a minoraxis (T1) passing through the medullary segment and intersecting theanterior (A) side, and a major axis (Y) perpendicular to the minor axis(T1) and intersecting the lateral side (L). The two segments are furtheroriented with respect to each other such that the two major axes form anacute angle (θ) therebetween. Preferably, the angle (θ) ranges fromabout 15 to about 50 degrees, most preferably about 32 degrees. Asdiscussed, the inventive stem preferably has a posterior concavity (29),which in this embodiment, as illustrated in FIG. 4B, is positioned atthe junction between these two segments.

The proximal region most preferably includes a lateral face that issteeply tapered in the medial direction, thus allowing theposterior/lateral corner (27) of the stem to lie closer to thelongitudinal axis (Z) of the stem, as illustrated, for example, in FIGS.3 and A--A through D--D, thereby resulting in minimal bulk in thisregion. Consequently, such lateral and posterior/lateral geometry allowsfor minimal bone removal from the greater trochanter during implantationas well as minimizing the enlargement of the implantation envelopeduring insertion of the stem. Preferably, the lateral face is tapered atangle (θ₂) ranging from about 5 degrees to about 14 degrees, mostpreferably about 8 degrees, as shown in FIG. 4, for example.

FIGS. 5 and 6 illustrate a left femoral component of the presentinvention seated within the intramedullary canal of the femur. FIG. 5Afurther illustrates the ability of the posterior concavity (29) to fitaround the calcar femorale (B¹), thereby minimizing its removal.

Referring now to FIGS. 1, 7 and 8, the top-portion (21) of the stemfurther includes neck (22a) which, in the preferred embodiment, receivesa modular femoral head component (not shown). The neck is inclined at anangle of approximately 2 degrees of anteversion in the sagittal plane(i.e. the anterior/posterior direction). In the coronal plan, the neckis positioned at an angle (θ₁) relative to the longitudinal axis (Z)ranging from about 45 degrees to about 55 degrees, more preferably about48 degrees, as shown in FIG. 1.

As shown in FIGS. 7 and 8, the shape of the proximal region is such thatthe dimension (V) of the coronal plane (from medial face to lateralface) is larger than the dimension (U) in the sagittal plane (fromanterior face to posterior face). Such relative dimensions increase therange of motion of the femoral head approximately 10 degrees per sidewhich totals 20 degrees in the anterior and posterior direction.Dimension (V) preferably ranges from about 1 inches to about 2 inches.Dimension (U) preferably ranges from about 0.5 inches to about 1 inch.!A preferred femoral head for use with the present invention is a V-40head manufactured by Howmedica, Inc., which has a 5°-40' taper rangingfrom -4 mm offset to +16 mm offset in multiple head diameters. Theforegoing anterior/posterior and medial/lateral dimensions (U,V) allowfor the use of the V-40 heads as well as similarly designed headswithout necessitating a skirt.

Preferably the proximal region includes a circumferential porous coatingas indicated by the shaded portions in FIGS. 1, 3, 5, 6, and 8, forexample. This composition and method of application to the femoral stemis taught, for example, in U.S. Pat. No. 4,550,448 (Kenna), which isincorporated herein by reference. The porous coating is designed topromote bony ingrowth, which in turn creates a bio-seal to prevent themigration of particulate debris.

II. Mid-stem Region

It is recognized in the art that providing a twist in the stem allowsthe stem to conform to the shape of the femoral cavity more readilyduring implantation. Certain conventional femoral stems comprise a twistthat occurs in the proximal region or substantially throughout theentire length of the stem. A disadvantage in having a twist in theproximal region of the prosthesis is that, upon implantation, the axialrotation of the stem that occurs due to the twist contributes to theenlargement of the anterior and posterior walls of the femoral cavityand subsequent formation of gaps between the stem and the bone. In thepresent invention, this problem is alleviated by restricting the twistto the mid-stem region of the stem where a tight cortical fit is notgenerally present. This allows the stem to be seated in the femoralcanal with minimal rotation during terminal stages of seating.

As illustrated in FIGS. 1 and 3, for example, the mid-stem region (30)of the hip stem is generally the region between cross-sectional linesF--F to J--J. Preferably, the length of the mid-stem ranges from about 2inches (for a smaller stem) to about 2.5 inches (for a larger stem). Inthe present invention, the stem has a twist extending throughout themid-stem region, beginning generally at the distal boundary (6) of themid-stem region (i.e. at cross-section line J--J), extending in theproximal direction, and ending at the proximal boundary (7) of themid-stem region (i.e. at cross-section line F--F) to provide a constanttwist angle (t) at the proximal boundary relative to the distal boundarywhich is consistent throughout the proximal region of the stem.

The twisting of the stem is best illustrated in FIG. 3 and FIGS. A--Athrough J--J. As shown, cross-sections A--A through J--J are takenperpendicular to the longitudinal axis (Z) of the stem. Line (1) isdrawn from the distal center (5) to the medial-most point of eachcross-section. In proceeding from the distal end beginning at sectionJ--J to the proximal region, the stem twists about the longitudinal axis(Z) of the stem such that the medial-most point shifts anteriorly.Broken line (2) represents the original line from the distal center (5)to the medial most point of the distal region (40). Thus, forcross-sections distal to section J--J, line (1) and line (2) areidentical. The twisting of the stem continues in the proximal direction,and is illustrated in the remaining cross-sections J--J through F--F,where the angle (t) between the original line (2) and line (1)represents the twist angle for that cross-section. Preferably, the angle(t) of twisting throughout the mid-stem region ranges from about 0.50degrees to about 8 degrees, most preferably from 3 degrees to 5 degrees.For example, in one preferred embodiment, the angle (t) in cross-sectionJ--J is about 1 degree, in cross-section I--I, about 1.75 degrees, incross-section H--H 2.75 degrees, in cross section G--G about 3.5degrees, and in cross-section F--F about 4 degrees. The actual twistingends at the proximal boundary, such that the angle (t) at cross-sectionsA--A through E--E (i.e. the proximal region) remains constant at 4degrees, for example, relative the distal boundary.

III. Distal Region

The distal region (40) of the stem most preferably has a length rangingfrom about 1.2 to about 2 inches. Referring now to FIGS. 8-10, theinventive stem (10) comprises an internal slot (41) extendinglongitudinally through the distal end and along the distal (i.e.longitudinal) axis (Z) in order to reduce bending stiffness. Unlike someconventional "straight slot" designs, wherein bending is reduced only inthe coronal plane, the present invention allows for a further reductionof bending stiffness in both the sagittal and coronal planes, therebyimparting more flexibility in the direction of the gait and stairclimbing distal tip loads. Specifically, the inventive stem comprises adistal internal slot (41) that is rotated at an acute angle (b) to thecoronal plane to cut the distal end into two separate tines, preferablya posterior/medial tine (44a) and an anterior/lateral tine (44b), asshown in FIGS. 8-10, for example. This angle (b) is defined by theintersection between an axis (42) that lies within the coronal plane andan axis (43) positioned centrally within the slot (41). Preferably, theangle of rotation (b) ranges from about 15 degrees to about 60 degrees,most preferably about 30 degrees. The length (L4) of the slot rangesfrom about 0.50 inch to about 2 inches, most preferably about 1 inch.

The distal region (40) further includes a distal end (40a) that ispreferably elliptically tapered, as shown in FIGS. 8 and 11, forexample. This distal end design functions to distribute the stresstransferred to the femur over a larger area, and further decreases themagnitude of stress at any particular location. The ellipticalconfiguration includes a semi-major diameter (d1) that is preferablyabout three times the length of the semi-minor diameter (d2).

The foregoing slot (41) is most preferably employed in stems having adistal diameter of greater than 11 mm (i.e. about 0.43 inches).Preferably, the width (W) of the slot is from about 0.09 inch to about0.170 inch, most preferably about 0.125 inch.

The distal region may further include a groove (45) on the outer surfaceof one or more of the tines, most preferably both, to aid in reducingstiffness of the implant. As illustrated in FIG. 10, the groove (45) hasa radius (R4) ranging from 0.1 to about 0.13 inches. The two radii (R5)of the remaining portion of each tine (44a, 44b) range in size of fromabout 0.10 inch to about 0.14 inch.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, and materials, as well as in the details of the illustratedconstruction may be made without departing from the spirit of theinvention.

We claim:
 1. A prosthetic femoral hip stem comprising:(a) a longitudinalaxis; (b) a distal region having a distal portion and a center, saidlongitudinal axis intersecting said center; (c) a proximal region; and(d) a mid-stem region positioned between said distal and proximalregions; wherein at least one cross-section taken perpendicular to saidlongitudinal axis through said proximal region includes: (e) asubstantially arcuate anterior side having a varying radius ofcurvature; (f) a posterior side having a concavity; (g) a substantiallyarcuate medial side having a radius of curvature; (h) a substantiallyarcuate lateral side that slopes anteriorly to define an acute angle ofdeclination; and (i) a substantially arcuate posterior/lateral cornerhaving a radius of curvature larger than said medial side radius ofcurvature.
 2. The stem of claim 1, wherein said angle of declinationranges from about 10 degrees to about 30 degrees.
 3. The stem of claim1, wherein said posterior concavity has a depth of from about 0.1 toabout 3.0 mm.
 4. The stem of claim 1, wherein said cross section furtherincludes a substantially arcuate posterior/lateral corner having aradius of curvature larger than said medial side.
 5. The stem of claim1, wherein said cross section further includes a transition regionconnecting the posterior and medial sides, wherein said transitionregion has a radius of curvature larger than said medial side.
 6. Thestem of claim 1, wherein said stem further includes a twist about thedistal axis.
 7. The stem of claim 6, wherein said twist generally beginsat a distal boundary of said mid-stem region and ends at a proximalboundary of said mid-stem region.
 8. The stem of claim 7, wherein saidtwist in the mid-stem region is from about 0.50 degrees to about 8.0degrees.
 9. The stem of claim 1, wherein said distal region furtherincludes an internal slot extending longitudinally through said distalend and along said distal axis.
 10. The stem of claim 9, wherein across-section of said distal region taken perpendicular to said distalaxis includes a first axis passing through said section in themedial/lateral direction and a second axis positioned centrally withinsaid slot and perpendicular to said distal axis; wherein said first andsecond axes define an acute angle at said distal center to cut saiddistal region into two separate tines.
 11. A prosthetic femoral hip stemcomprising:(a) a longitudinal axis; (b) a proximal region; (c) a distalregion; and (d) a mid-stem region positioned between said distal andproximal regions; wherein at least one cross-section taken perpendicularto said longitudinal axis through said proximal region includes: (e) asubstantially arcuate anterior side having a varying radius ofcurvature; (f) a posterior side having a concavity; (g) a substantiallyarcuate medial side having a radius of curvature; (h) a substantiallyarcuate lateral side that slopes anteriorly to define an acute angle ofdeclination; (i) a substantially arcuate posterior/lateral corner havinga radius of curvature larger than said medial side radius of curvature;and (j) an axis bisecting said cross section through said anterior andposterior sides an equidistance between said lateral and medial sides todefine a first section including said lateral side and a second sectionincluding said medial side, said first section having a larger area thansaid second section.
 12. The stem of claim 11, wherein said angle ofdeclination ranges from about 10 degrees to about 30 degrees.
 13. Thestem of claim 11, wherein said posterior concavity has a depth of formabout 0.1 mm to about 3.0 mm.
 14. The stem of claim 11, wherein saidcross section further includes a transition region connecting theposterior and medial sides, wherein said transition region had a radiusof curvature larger than said medial side radius of curvature.